Referring to FIGS. 1a and 1b, a typical backplane and card arrangement of the prior art is shown. In the arrangement 10, a plurality of electrical circuit cards 12 are mounted on a backplane 14 and interconnected to leads in the backplane 14 through a plurality of slots 26. The backplane 14 may comprise a multi-layer printed circuit. Often in this and similar arrangements several data lines and/or control lines are common to two or more cards 12. For instance, there may be an input/output card 16, a processor card 18, a memory card 20 and some type of data manipulation card 22 all sharing common data lines. The data lines 24 are shown interconnecting the various cards 12.
In this arrangement, signals may be sent back and forth between cards 12 on the data lines 24. The signals transmitted on these data lines 24 are transmitted at high speeds. In addition to data lines 24, several other signals may traverse the backplane 14 at any given instance of time. To maximize the use of backplane space, data and control lines are located in close proximity to one another.
Two common problems associated with the transmission of electrical signals in lines that are located in close proximity to one another are shielding and cross talk. Both of these phenomena are well known in the art. Shielding refers to establishing a ground plane around a signal line to insulate it from outside influences, e.g., coaxial cable. Cross talk refers to the occasion when a signal in one line interferes with a signal in another adjacent transmission line through inductive coupling.
High speed high frequency signal transmission makes possible the transmission of electrical signals at rates in the MegaHertz frequency transmission range. However, these high frequency signals accentuate the problems.
To provide some form of shielding protection and to provide a common ground, a backplane 14 is configured with a common ground plane. The ground plane may either be internal or external to the backplane 14 or both. Usually it is external, thereby offering some shielding to the backplane as a whole. A significant problem with this arrangement is that it does not provide shielding between leads or wires in the backplane, but rather protects the backplane as a whole from external signals.
Turning now to crosstalk, the phenomenon of inductive cross talk is well known in the data communications industry, particularly with respect to telephones. Cross talk on open-wire carrier facilities (e.g., telephone poles) results from the inductive couplings which exists between parallel wire pairs. This cross talk can be reduced to tolerable dimensions by a method of line treatment known as transposing in which the positions of the wires of each pair are interchanged (transposed) at intervals along the length of the facility.
Since, in the telephone scenario, it is practically impossible to space each wire of a pair equally distant from all disturbing conductors, the next best thing is to arrange the wires so that they "take turns" in sharing positions nearer and farther from disturbing conductors. This is done by transposing the wires systematically. Transpositions must be designed to cancel crosstalk locally, rather than around the whole circuit, so that phase shifts won't partially or completely cancel the effect of the transpositions. A common way of achieving this result in phone lines is twisting the individual lines and their return lines in bundles which are further twisted circularly about themselves.
This twisting of signal pairs, however, has not been utilized in backplanes 14. Alternatively, in backplanes, which principally stem from digital electronics, digital components are used to handle the effects of cross talk. These components can take up a significant amount of backplane real estate, be unnecessarily costly and produce heat. Secondly, with respect to telephone lines, backplanes are etched printed circuits, thereby eliminating the freedom of movement that enables telephone lines to be twisted. Occasionally, a return line may be added externally to a backplane, but no twisting (field cancellation) occurs.
The substantial intricacies of providing shielding and twisted pairs in an etched backplane have not been tackled in the prior art. They form the basis of the present application.